(1) The point of a file, being thinner than the middle, is liable to become hot sooner, and attain a purple colour before the thickest part shows any degree of heat; to prevent which the heat must be applied gradually, by shifting the file backwards and forwards in front of (not over, because the smoke will colour it) a bright fire or furnace door, so as to keep the thickness. A piece of sheet - iron should be hung in front just above the file, to keep the glare of the fire from the operator's eyes, and by comparison from time to time with a new file (which should be at hand) he will be enabled to judge of the progress of the operation, and regulate the heat until the desired colour is obtained, when the file must be immediately plunged into water. Pale straw colour is low enough for most files, but no file should be brought lower than deep straw. (2) Cover with oil, and hold over the fire until the oil blazes, and as soon as the flame runs all over the file, plunge it in the water; or put in a moderate hot oven for 1/2 hour if large file; but if small, the former is better. (3) A mixture of tallow, hog's lard, and arsenic makes an excellent mixture for hardening files.